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Guerre info-centrée et recomposition du champ de bataille ; quels impacts pour la conception des besoins capacitaires
No full textInternational audiencePar-delà l’émergence de nouvelles capacités militaires avec, en tête, le foisonnement des drones en Ukraine, le champ de bataille se recompose rapidement autour de l’usage des données. Tout laisse envisager que cette évolution devrait s’accentuer dans les prochaines années sous la double influence des leçons tirées des engagements, en particulier en Ukraine, et de l’évolution des possibilités technologiques, en particulier le déploiement de solutions d’intelligence artificielle.Le champ de bataille tend à se confondre de plus en plus avec un espace de données qui peuvent être collectées (pour assurer la transparence du champ de bataille), exploitées en quasi temps réel (pour dominer l’adversaire) ou encore permettre une plus grande efficacité de l’emploi des capacités et des troupes déployée (notamment en nourrissant des algorithmes d’intelligence artificielle afin de réduire le « brouillard de la guerre »).Si toutes les nouvelles capacités appuient leur efficacité sur une consommation massive de données, il est important de s’interroger sur les transformations structurelles qui ne sont pas nécessairement perceptibles aujourd’hui, mais qui peuvent conduire à repenser la conception des capacités militaires. En effet, dans quelle mesure l’accès et l’exploitation des données remettent-ils en question le développement capacitaire ? En effet, les principaux pays producteurs d’armes ont concentré leurs efforts depuis un demi-siècle sur l’amélioration de plateformes majeures dont le coût devient insoutenable, confirmant la Loi d’Augustine, mais qui arrivent aujourd’hui à certaines limites. De plus, l’intégration croissante de fonctionnalités sur des plateformes de plus en plus complexes en nombre de plus en plus réduit rend intenable cette trajectoire.La question proposée dans cette contribution est d’analyser dans quelle mesure la place croissante des données dans les opérations militaires entraîne une remise en question de cette approche de l’efficacité militaire et donc de la conception des capacités militaires dans leur définition et leur rapport à l’usage des données
From citizen science to AI models: Advancing cetacean vocalization automatic detection through multi-annotator campaigns
No full textInternational audienceContinuous underwater Passive Acoustic Monitoring (PAM) has emerged as a strong tool for cetacean research. To handle the vast volume of collected data, it is essential to employ automated detection and classification methods. The recent advancement of deep learning, involving model training and testing, requires a large amount of labeled data. These labels are derived through the manual annotation of audio files often reliant on human experts. Based on an annotation campaign focusing on blue whale calls in the Indian Ocean involving 19 novice annotators and one expert in bioacoustics, this study explores the integration of novice annotators in marine bioacoustics research, through citizen science programs, which could drastically increase the size of labeled datasets and enhance the performance of detection and classification models. The analysis reveals distinctive annotation profiles influenced by the complexity of vocalizations and the annotators' strategies, ranging from conservative to permissive. To address the challenges of annotation discrepancies, Convolutional Neural Networks (CNNs) are trained on annotations from both novices and the expert. The results show variations in model performance. Our work highlights the importance of annotation guidelines encouraging a more conservative approach to improve overall annotation quality. In an effort to optimize the potential of multi-annotation and mitigate the presence of noisy labels, two annotation aggregation methods (majority voting and soft labeling) are proposed and tested. The results demonstrate that both methods, particularly when a sufficient number of annotators are involved, significantly improve model performance and reduce variability: the standard deviation of the area under PR and ROC curves fall under 0.02 for both vocalizations with 13 aggregated annotators, while it was at 0.17 and 0.21 for the Blue Whale Dcalls and 0.05 and 0.04 for the SEIO PBW vocalizations with all annotators separately. Moreover, these aggregation methods enable the training of models using non-expert annotations that achieve performance of models trained with expert annotations. These findings suggest that crowdsourced annotations from novice annotators can be a viable alternative to expert annotations
Augmentation du courant moyen des accélérateurs laser plasma pour applications industrielles
No full textThis doctoral thesis is part of a CIFRE collaboration between Thales-MIS and the Laboratoire d'Optique Appliquée (LOA). The main objective is to enhance the average current of low-energy laser-plasma accelerators, particularly in the range of a few MeV. This advancement is particularly interesting for low-energy applications such as industrial X-ray tomography, which does not require monoenergetic electron beams.Experiments were conducted using the 60,TW laser system installed in the Salle Jaune at LOA, capable of generating 30 fs pulses. Through meticulous exploration of plasma densities, laser energies, gas targets, and focusing degrees, we identified conditions conducive to producing highly divergent electron beams (i.e., >100 mrad) at energies of a few MeV, with charges ranging from 5 to 30 nC. We also achieved a maximum laser-to-electron energy conversion efficiency of approximately 14 %, one of the highest ever measured. Looking ahead to future laser systems capable of achieving average powers of around 100 W, these configurations could pave the way for generating laser-plasma accelerated electron beams with average currents exceeding 1 microampere, surpassing the current state of the art in laser-plasma accelerators. To facilitate these innovative experiments, we designed a supersonic glass nozzle and permanent magnetic dipoles to deflect electrons towards scintillating screens for beam spectroscopy. Concurrently with the experiments, this thesis also delved into Particle-In-Cell (PIC) simulations to study acceleration mechanisms. Using a dedicated numerical tool for processing PIC simulation results, we demonstrated that the ponderomotive force of the laser plays a predominant role in electron acceleration. Notably, the majority of particles are not injected into plasma waves but rather slide along the laser pulse, thereby gaining low energies on the order of a few MeV.Cette thèse de doctorat s'inscrit dans le cadre d'une collaboration CIFRE entre Thales-MIS et le Laboratoire d'Optique Appliquée (LOA). L'objectif principal est d'améliorer le courant moyen des accélérateurs laser-plasma à faible énergie, notamment dans la gamme de quelques MeV. Cette avancée revêt un intérêt particulier pour les applications à faible énergie telles que la tomographie industrielle par rayons X, ne nécessitant pas de faisceaux d'électrons monoénergétiques.Des expériences ont été menées au moyen du système laser de 60 TW installé dans la Salle Jaune du LOA, capable de générer des impulsions de 30 fs. À travers une exploration minutieuse des densités de plasma, des énergies laser, des cibles gazeuses et des degrés de focalisation, nous avons identifié les conditions propices à la production de faisceaux d'électrons hautement divergents (i.e., >100 mrad) de quelques MeV, avec des charges variant de 5 à 30 nC. Nous avons également atteint une efficacité maximale de conversion d'énergie laser-électron d'environ 14 %, parmi les plus élevées jamais mesurées. En envisageant les futurs systèmes laser capables d'atteindre des puissances moyennes d'environ 100 W, ces configurations pourraient ouvrir la voie à la réalisation de faisceaux d'électrons accélérés par laser-plasma, avec des courants moyens dépassant 1 microampère, surpassant ainsi l'état de l'art actuel des accélérateurs laser-plasma. Pour mener à bien ces expériences novatrices, nous avons conçu une buse supersonique en verre et des dipôles magnétiques permanents permettant de dévier les électrons vers des écrans scintillants pour effectuer la spectrométrie des faisceaux produits. Parallèlement aux expériences, cette thèse a également approfondi les simulations Particle-In-Cell (PIC) pour étudier les mécanismes d'accélération. Grâce à un outil numérique spécifiquement développé pour traiter les résultats des simulations PIC, nous avons démontré que la force pondéromotrice du laser joue un rôle prépondérant dans l'accélération des électrons. Notamment, la majorité des particules ne sont pas injectées dans les ondes du plasma, mais glissent plutôt sur l'impulsion laser, acquérant ainsi une faible énergie de l'ordre de quelques MeV
Dispersion and ellipticity of Rayleigh waves in a soil substrate supporting resonant beams and plates
No full textInternational audienceThe behavior of surface waves in a soil supporting an array of beams in three dimensions, or an array of plates in two dimensions, with compressional and flexural resonances is examined both theoretically and numerically. Our findings demonstrate that Love waves, characterized by displacements perpendicular to the sagittal plane, can propagate even without a homogeneous guiding layer, owing to the influence of flexural resonances in beams. Within the sagittal plane, hybridized Rayleigh waves exhibit a dispersion that is notably altered by the presence of the array, with their properties emerging from the interaction between flexural and compressional resonances. Notably, we uncover the coexistence of two Rayleigh waves with distinct wave numbers within specific frequency ranges, corresponding to prograde and retrograde motions. Additionally, both waves significantly amplify ground motion, either horizontally or vertically. Similar physics, yet quantitatively different, is demonstrated in the case of plate arrays
Differential pumping for kHz operation of a Laser Wakefield accelerator based on a continuously flowing Hydrogen gas jet
No full textLaser-Wakefield Accelerators (LWFA) running at kHz repetition rates hold great potential for applications. They typically operate with low-energy, highly compressed laser pulses focused in high-pressure gas targets. Experiments have shown that the best-quality electron beams are achieved using Hydrogen gas targets. However, continuous operation with Hydrogen requires a dedicated pumping system. This work presents the design of a differential pumping system, enabling, for the first time, continuous operation of our kHz LWFA using a high-pressure Hydrogen gas jet. The system successfully maintained a pressure below 3e-4 mbar, even with a free-flowing gas jet operating at 140 bar backing pressure. Numerical fluid dynamics and optical simulations were used to guide and validate the system's design
Coupling of discontinuous Galerkin and pseudo-spectral methods for time-dependent acoustic problems
No full textInternational audienceMany realistic problems in computational acoustics involve complex geometries and sound propagation over large domains, which requires accurate and efficient numerical schemes. It is difficult to meet these requirements with a single numerical method. Pseudo-spectral (PS) methods are very efficient, but are limited to rectangular shaped domains. In contrast, the nodal discontinuous Galerkin (DG) method can be easily applied to complex geometries, but can become expensive for large problems.In this paper, we study a coupling strategy between the PS and DG methods to efficiently solve time-domain acoustic wave problems. The idea is to combine the strengths of these two methods: the PS method is used on the part of the domain without geometric constraints, while the DG method is used around the PS region to accurately represent the geometry. This combination allows for the rapid and accurate simulations of large-scale acoustic problems with complex geometries, but the coupling and the parameter selection require great care.The coupling is achieved by introducing an overlap between the PS and DG regions. The solutions are interpolated on the overlaps, which allows the use of unstructured finite element meshes. A standard explicit Runge-Kutta time-stepping scheme is used with the DG scheme, while implicit schemes can be used with the PS scheme due to the peculiar structure of this scheme. We present one-and two-dimensional results to validate the coupling technique. To guide future implementations of this method, we extensively study the influence of different numerical parameters on the accuracy of the schemes and the coupling strategy
Experimental Investigation of Repetitive Femtosecond Laser Energy Deposition in a M=3 Supersonic Airflow
No full textInternational audienceA picosecond 200-mJ pulsed laser operating at 1 kHz is used to create large aspect ratio heated channels in front of a blunted test model in a M=3 flow. The energy deposition induces a significant disturbance of the flow and large fluctuations of the drag signal. The heated region extends over 50-80 mm upstream of the test item. For this given repetition rate, the raw drag balance signal shows no significant decrease in the averaged drag, which could be due to the vibration induced by the fluctuating drag on the mechanical system
Accelerated iterative DG finite element solvers for large-scale time-harmonic acoustic problems
No full textInternational audienceFinite element methods are widely used to solve time-harmonic wave propagation problems, but solving large cases can be extremely difficult even with the computational power of parallel computers. In this work, the linear system resulting from the finite element discretization is solved with iterative solution methods, which are efficient in parallel but can require a large number of iterations. In standard discontinuous Galerkin (DG) methods, the numerical solution is discontinuous at the interfaces between the elements. In hybridizable DG methods, additional unknowns are introduced at the interfaces between the finite elements, and the physical unknowns are eliminated from the global system, resulting in a hybridized system. We have recently proposed a new strategy, called CHDG, where the additional unknowns correspond to transmission variables, whereas in the standard approach they are numerical fluxes. This strategy improves the properties of the hybridized system for faster iterative solution procedures. In this talk, we present and study a 3D CHDG implementation with nodal finite element basis functions. The resulting scheme has properties amenable to efficient parallel computing. Numerical results are presented to validate the method, and preliminary 3D computational results are proposed
Stress ratio effect on fatigue crack growth assessment via thermoelastic stress analysis
No full textInternational audienceThe fatigue performance of welded joint is strongly dependent on welding residual stresses as well as the local geometry of the weld toes. To ensure the structural integrity over time of such components, fine predictive models must be proposed together with efficient experimental methods for their assessment. This study introduces a model based on linear elastic fracture mechanics to forecast fatigue life in service of high strength steel welded joints. This model considers the effect of the local stress ratio (dependent on stabilized residual stresses) on the crack growth rate through the definition of an effective stress range. The use of Thermoelastic Stress Analysis (TSA) is used to perform in-situ monitoring of fatigue cracks on welded joints to experimentally characterize the crack-closure phenomenon at different load ratios. Indeed, under adia batic conditions, the temperature's first harmonic at the surface of a structure submitted to cyclic loading is proportional to the stress tensor's first invariant's amplitude. As a result, the presence of a surface crack and the non-linearity induced by the alterative opening and closing within one loading cycle can be observed in the immediate temperature response. This phenomenon is used to evaluate a crack-opening rate that is related to the fatigue life of the welded joints
Reversible photoregulation of G-quadruplex DNA structures by non-covalent azobenzene derivatives
No full textInternational audienceG-quadruplexes (G4) are non-canonical DNA structures involved in important cell regulatory functions associated with their folding mechanism. The design of small ligands capable of modulating their formation/stabilization is therefore of growing interest for the development of new anti-cancer therapies. In particular, the reversible control of G4s using bistable photoswitches offers promising perspectives for applications in photopharmacology and DNA nanotechnology, but remains largely unexploited [1]. It has long been demonstrated that the folding/unfolding of human telomeric (HT) G4 sequences can be induced by azobenzene-derived photoswitches [2]. However, the dynamics and mechanisms underlying these processes have never been investigated. Here we present a comprehensive study of complexes made of non-covalent azobenzenes bearing quaternary ammonium substituents (AZO) with different G4 sequences, by using a combination of stationary and time-resolved optical and chiroptical spectroscopic methods. This study revealed a non-cooperative binding mode of AZO with HT G4 sequences of the type 5'-GGG(TTAGGG)3-3' and the thrombin-binding aptamer G4 sequence, 5'-GGTTGGTGTGGTTGG-3' (TBA), in the absence of physiological cations. The binding of AZO to DNA induces the formation of parallel G4 topologies that can be reversibly unfolded under UV/visible excitation without noticeable fatigue. Femtosecond transient absorption measurements show that the isomerization of AZO is slowed by a factor of 4 in the presence of G4 (62ps vs. 16ps), while millisecond time-resolved circular dichroism provides evidence that G4 unfolding takes place within a few tens of milliseconds [3]